Astronomers Find 100,000-light-year Bow Shock in the Milky Way’s Outskirts

What’s invisible except in a handful of wavelengths and extends across 100,000 light-years? According to new simulations and a new look at archival data, the answer to this riddle is a humongous bow shock in the Milky Way’s outskirts.

If you’ve ever seen a fighter jet fly (whether in real life or in Top Gun), you know that when the plane overtakes the speed of sound, it creates a shock wave that encompasses and trails behind the plane. The sudden change in pressure accompanying the shock wave is known as a sonic boom — like when Top Gun’s Maverick “buzzed the tower” and spilled the officer’s coffee.

Something similar, minus the coffee, is happening in the bath of hot, sparse gas that surrounds the Milky Way Galaxy. The largest satellite, the Large Magellanic Cloud (LMC), is falling into this gas faster than the local speed of sound (that is, the speed at which pressure waves can propagate). In the sparse gas, the speed of sound is 165 kilometers per second (370,000 mph). The LMC is crashing through this medium at almost twice that speed (320 km/s, or Mach 2.)

Astronomers have long suspected that the LMC’s infall should create a bow shock. Now, David Setton (Princeton University) and colleagues have simulated a “wind tunnel” to predict how large it should be. In this computer simulation, the LMC is a bundle of gas and stars that sits at rest in a box.

The gas around the Milky Way rushes like a headwind toward and around this bundle. While the stars ignore the headwind, passing straight through it, the gas pushes into the wind to generate a bow shock that extends three times the size of the dwarf galaxy.

With an idea of what to look for, and where, Setton and colleagues then searched recent measurements taken by the Wisconsin H-alpha Mapper (WHAM). The hydrogen-alpha emission that WHAM is mapping marks the presence of hot gas. Projecting WHAM data onto their shock simulation, the researchers found good — though not perfect — alignment. They’ve posted these results on the arXiv astronomy preprint server.

Marcel Pawlowski (Leibniz-Institute for Astrophysics, Germany), who was not involved in the work, thinks it’s likely a bow shock is there, but he also notes that more work is needed. “The morphological match isn’t that clear,” he says. “However, I expect that more sensitive data and a quantitative comparison would allow [us] to evaluate this better, so the paper can be a good motivation for such follow-up observations.”

The team acknowledges that the simplified “wind tunnel” model is a first step, and more detailed work is already underway.

Studying the bow shock will help astronomers understand the behavior of the gas around the Milky Way. Sparse as it is, there’s still a lot of it around our galaxy, and it serves as a reservoir for future star formation in the disk. It’s also the dumping ground of gas fountains sprayed out by supernovae, and thus is continuously evolving.

As gas-rich dwarf galaxies such as the LMC and its smaller companion (the SMC) approach the Milky Way, they mix up this reservoir, bringing the whole soup up to a hotter, common temperature, team member Gurtina Besla (University of Arizona) explains. That might prevent gas from cooling and falling in toward the spiral disk. At the same time, she adds, “there might be other cooling processes that are not hindered by this effect though, so we need to study this better.”

The LMC and SMC are so close, Besla notes, that they’ve become “the perfect astrophysical laboratory” with which to study galactic dynamics and evolution, not to mention the role of dark matter, star formation, and stellar feedback on these processes.  “I think it is awe-inspiring to be working to understand this pair of galaxies that have been known to humans since we first looked to the sky,” she adds.

“It is a complex system with many moving and interacting parts,” Pawlowski agrees, “so it is good to see another, thus-far overlooked aspect being explored now.”